Currently, more than half of all clinical nuclear imaging procedures in the U.S. are cardiac imaging, and nearly all of these are stress/rest myocardial perfusion SPECT imaging. In addition to routine EKG-gated imaging protocols, many investigators are exploring the utility of dynamic and respiration-gated SPECT to assess cardiac function. These advanced imaging techniques demand higher performance than existing SPECT systems provide. The goal of our research addresses this demand by developing a high performance SPECT system optimized for cardiac imaging. Our research emphasis over the years stresses the development of innovative instrumentation that assures the quality of data acquired at the front-end followed by advanced software processing techniques to provide high-quality tomographic images. The design concept of the proposed new cardiac SPECT/CT hybrid platform, called MarCS, is evolved from our previously developed brain SPECT system. MarCS achieves high performance through efficient and effective detection of desirable photons;this performance will be complemented by an integrated and practical transmission imaging scheme. The ability to perform high-quality emission and transmission imaging on the same system sequentially facilitates MarCS to provide high-quality quantitative SPECT images with efficiency, practicality and consistency to clinical operations, and accuracy to diagnosis. Since the feasibilities of the MarCS platform have been established, it is time to develop a prototype system. Due to the huge magnitude and complexity of this project, we divide the prototype development into two 3-year phases: a design and an engineering phase. This application is for the funding of the design phase, which is actually more significant in the context of research and which will guide the next engineering phase to realize the full potential of the prototype. Our preliminary studies for the first implementation of these design concepts - MArCS-l system - are very encouraging;further optimization and experimental investigations are necessary prior to the construction of a full-scale laboratory prototype. We propose to verify the design concepts rigorously and optimize the design of MarCS, to achieve optimized performance, and a plan for the next phase - construction of a prototype. This project will expand the utility of cardiac SPECT imaging and promote its status as a complementary clinical diagnostic tool with x-ray CT and MR1.